Chapter 4

• Finish the sentence: When dissolved in water, an acid...

  releases H+ ions into the solution. These H+ ions are simply protons, and therefore acids are called proton donors
• Finish the sentence: All Acids contain ________ in their formulae

  hydrogen
• How to write the equation for when hydrogen chloride dissolves in water and disassociates into its component ions? (2)

  1) HCl(g) + aq -----> H+(aq) + Cl-(aq)

  
  

  
  

  2) aq is the symbol used to show that there is an excess of water present
• What is pH a measure of?

  How acidic or basic a solution is
• Can a solid have a pH?

  No - pH is a measure of the relative concentrations of H+ ions and OH- ions and only defined in an AQUEOUS SOLUTION
• Finish the sentence: Acids release...

  hydrogen ions when they are in an aqueous solution
• What is a base? (3)

  1) A base is a substance which reacts with an acid to produce a salt and water

  
  

  2) Hence a base neutralises an acid to form salt + water

  
  

  3) SOLUBLE bases have a pH of above 7
• Can you find the pH of an insoluble base?

  NO - Because it is not soluble and will not disassociate into its constituent ions
• What is an alkali?

  
  

  Show an equation for NaOH

  A base that dissolves in water (so its soluble) releasing OH- ions into the solution

  
  

  NaOH(s) + aq ----> Na+(aq) + OH-(aq)
• What is the relation between the concentration of H+ ion and pH?

  The higher the concentration of H+ ions the lower the pH
• How can you accurately calculate the pH of an aqueous solution?

  -log[H+ ion conc. in M]

  
  

  e.g. if H+ ion concentration was 1*10^(-5) then the negative log would be 5

  
  

  therefore pH would be -- pH 5
• Why does a soluble base increase pH?

  When a base is dissolved in water, the balance between hydrogen ions and hydroxide ions shifts, and more hydroxide ions are added to the solution

  
  

  Because the base reacts with hydrogen ions, the result is a solution with more hydroxide ions than hydrogen ions. This kind of solution is alkaline.

  
  

  Now there is an decreased proportion of H+ ions in the solution = increased pH
• What is an indicator?

  A dye that changes colour depending on whether it's above or below a certain pH
• What instrument can be used to measure pH?

  pH probe
• How does the colour of litmus change if it is in a:

  
  

  1) Acid

  2) Neutral solution

  3) Alkali

  1) Acid = RED

  2) Neutral = PURPLE

  3) Alkali = BLUE
• How does the colour of phenolphthalein change if it is in a:

  
  

  1) Acid

  2) Neutral solution

  3) Alkali

  1) Acid = COLOURLESS

  2) Neutral = COLOURLESS

  3) Alkali = PINK
• How does the colour of methyl orange change if it is in a:

  
  

  1) Acid

  2) Neutral solution

  3) Alkali

  1) Acid = RED

  2) Neutral = ORANGE

  3) Alkali = YELLOW
• What does acid strength tell us?

  What proportion of the acid molecules ionise in water
• What are the strong acids?

  
  

  Also give an equation for HCl

  Acids that ionise completely in aqueous solutions, so all of the acid particles disassociate to release H+ ions = LOW pH

  
  

  HCl(aq) --> H+(aq) + Cl-(aq)
• Finish the sentence: Mineral acids are...

  STRONG
• What are weak acids?

  
  

  And what is the special thing that happens with them?

  Acids that partially ionise in aqueous solutions, so not all of the acid molecules disassociate to release H+ ions

  
  

  This ionisation is a reversible reaction, and when it reaches equilibrium, both the acid and the constituent ions are made, HOWEVER THE POSITION OF EQUILIBRIUM LIES TO THE LEFT

  
  

  CH3COOH <----------> H+ + CH3COO
• What is concentration (of an acid)?

  How much acid there is in a certain volume of water

  
  

  the larger the number of acid molecules there are in a certain volume of liquid, the more concentrated the acid is
• Concentration vs Acid strength

  Concentration of acid = How many acid particles in a litre of water

  
  

  Strength = How many of those acid particles disassociate to make H+
• Finish the sentence: A dilute strong acid means...

  ALLof the acid particles release H+ ions, but there aren't that many acid particles in the solution in the first place
• Finish the sentence: A concentrated weak acid means...


  not all of the acid particles disassociate to release H+ ions, but there are a lot of acid particles in the solution
• Finish the sentence: The pH of an acid is dependent on...

  the acid's concentration, and therefore the concentration of H+ ions in the solution
• For a GIVEN concentration, why is the pH of a strong acid LOWER than the pH of a weak acid?

  1) A strong acid fully disassociates in water, meaning there is A HIGH concentration of H+ ions in the solution leading to a low pH

  
  

  2) A weak acid only partially disassociates so there is a lower concentration of H+ ions THAN IN A SOLUTION OF A STRONG ACID OF THE SAME ACID CONCENTRATION - this leads to a higher pH
• Finish the sentence: Acid concentration as well as H+ concentration is correlated to...

  pH
• Finish the sentence: Increasing H+ ion conc. x10 means..

  pH decreases by 1
• What does pH 7 really mean in detail?

  When the concentration of hydrogen ions is equal to the concentration of hydroxide ions
• What happens when acid added to water?

  Acid particles ionise/disassociate to produce H+ ions and another ion ( Cl- or NO3- etc...)
• What does it mean when an acid is neutralised?

  It means that H+ ions from an acid react with a base to form a salt and NEUTRAL water:

  
  

  The H+ ions are replaced by metal or ammonium ions (if there is any) from the base
• What is the ionic equation for neutralisation using a soluble base and an acid?


  H+(aq) + OH-(aq) ----> H2O(l)
• What happens when a metal oxide or hydroxide reacts with an acid?


  ONLY salt and water are produced

  
  

  CuO + CH3COOH ----> (CH3COO)2 Cu + H2O

  
  

  Everything except water (liquid) and CuO (solid) is aqueous in this example

  
  

  See how the H from the -COOH group is replaced by the Cu 2+ ????
• What happens when an alkali reacts with an acid?

  1) Both reactants are in solution, and the overall reaction produces a salt and water as well

  
  

  2) An alkali is a soluble base so it relaease OH- ions in water

  
  

  3) So only IN THIS CASE can we write:

  
  

  H+(aq) + OH-(aq) -----> H2O(l)
• Finish the sentence: Acid + metal carbonate?

  Salt + water + Co2
• What so cool about sulfuric acid? (extension) (4)

  1) H2SO4 is a strong acid. It has 2 hydrogen atoms.

  
  

  2) However when H2SO4 disassociates, each molecule only releases ONE of its two hydrogen atoms as a H+ ion:

  
  

  H2SO4(aq) -----> H+(aq) + HSO4-(aq)

  
  

  [this ion is the hydrogen sulfate ion]

  
  

  3) The resulting hydrogen sulfate ions then onlypartiallydisassociate:

  
  

  HSO4-(aq) <-------> H+(aq) + SO4 2-(aq)

  
  

  SO FIRST SULFURIC ACID BEHAVES LIKE A STRONG ACID

  
  

  THEN THE HYDROGEN SULFATE IONS BEHAVE AS A WEAK ACID (EVEN THOUGH IT ISN'T ONE)
• What is a titration?

  It is a technique used to accurately measure what volume of one solution is needed to react completely with a certain volume of another solution
• What can a titration be used for?

  1. Finding the concentration of a solution

  
  

  2. Identification of unknown chemicals

  
  

  3. Finding the purity of a substance
• What is oxidation number?


  A number assigned to an element in chemical combination which represents the number of electrons lost (or gained, if the number is negative), by an atom of that element in the compound.